Transimpedance amplification apparatus with source follower structure

ABSTRACT

A transimpedance amplification apparatus includes a signal source for generating a current signal, a source follower stage, a common source stage and a shunt feedback resistor. The source follower stage having a source follower structure receives the current signal to reduce an impedance of the signal source. The common source stage, following the source follower stage, driven by the reduced signal source impedance, amplifies the current signal to extend a frequency bandwidth of the current signal and buffers the amplified signal with the extended frequency bandwidth thereof maintained, wherein the reduced signal source impedance serves to extend a frequency bandwidth of the common source stage. The shunt feedback resistor, which is installed between the source follower stage and the common source stage, adjusts an input DC bias of the source follower stage and increasing a transimpedance gain of the common source stage.

FIELD OF THE INVENTION

[0001] The present invention relates to a transimpedance amplificationapparatus in an optical communications system; and, more particularly,to a transimpedance amplification apparatus with a source followerstructure for extending a frequency bandwidth thereof without additionalpassive devices.

BACKGROUND OF THE INVENTION

[0002] Recently, ultrahigh-speed data communications techniques, e.g.,an optical communications technique using an optical fiber, haveadvanced rapidly, but demand for transmission of a growing amount ofdata keeps getting stronger, too. To transmit a larger amount of data,it is necessary to implement a broadband amplification system operatingat a wider frequency bandwidth. For this purpose, an active elementcapable of stably operating in an ultrahigh frequency band has to bedeveloped. Moreover, it is also important to know how to install a newlydeveloped element and others in a broadband amplifier circuit, i.e., toimprove the way to design a broadband amplifier circuit in order to makethe newly developed elements function effectively. The front-end of anoptical communication receiver includes a photodetector, which convertsan optical signal into an electrical current signal, and a preamplifierfor extending a frequency bandwidth.

[0003] One of key components in the broadband amplification system isthe preamplifier. A common source transimpedance amplifier (TIA) isusually used as the preamplifier.

[0004]FIG. 1 shows a circuit diagram of a conventional common source TIA100, which includes a plurality of transistors 10, 20, 24 and 30, amultiplicity of resistors 12, 14, 18, 22, 26, 28 and 32 and a powersupply 16.

[0005] The conventional common source TIA 100 amplifies a current signalinputted thereto by means of four-part transistors 10, 20, 24 and 30,thereby outputting a voltage signal. That is, the transistor 10, a firstamplifying transistor, basically amplifies the current signal and thetransistor 20, a first buffer transistor, buffers the amplified signal.Thereafter, the transistor 24, a second amplifying transistor,secondarily amplifies the buffered signal from the first buffertransistor 20 and then the transistor 30, a second buffer transistor,buffers the secondarily amplified signal.

[0006] A wide frequency bandwidth is one of the most importantcharacteristics to be considered in designing a high-speed opticalcommunications system, especially, TIA. However, it is not easy toextend a frequency bandwidth of the conventional common source TIA asmuch as required by the optical communications system. Accordingly, manyschemes, e.g., gain peaking techniques using passive devices ofinductors or capacitors, have been proposed.

[0007] A shunt inductive-peaking scheme, one of the most frequently usedtechniques, connects an inductor to a drain of the first and/or thesecond amplifying transistor in the conventional common source TIA 100,resulting in a resonance with parasitic capacitances. Although the shuntinductive-peaking scheme extends the frequency bandwidth, straycapacitances of on-chip-inductor often cause a bad influence uponbandwidth increment. The shunt inductive-peaking scheme has furtherproblem in that self-resonant frequency (SRF) and Q-value of theinductor greatly limit on high-frequency applications.

[0008] Another method for wideband design is a capacitive-peakingscheme, in which a capacitor is coupled to the resistor 22, in parallelat the source of the first buffer transistor 20 of the conventionalcommon source TIA 100. In the result, an extra pole is added to atransfer function of the conventional common source TIA 100 so that thefrequency bandwidth may be extended. However, this method also hasserious problems due to the variation of capacitance caused by parasiticpad capacitance and process variation.

[0009] Therefore, it is desirable to develop a TIA capable of extendingthe frequency bandwidth without regard to SRF and Q-value of an inductorand simultaneously less sensitive to parasitic pad capacitance andprocess variation.

SUMMARY OF THE INVENTION

[0010] It is, therefore, an object of the present invention to provide atransimpedance amplification apparatus of a source follower structurecapable of extending the frequency bandwidth by reducing sourceimpedance in an input node without regard to SRF and Q-value of aninductor and simultaneously less sensitive to parasitic pad capacitanceand process variation.

[0011] In accordance with the present invention, there is provided atransimpedance amplification apparatus with a signal source forgenerating a current signal, which includes: a source follower stagehaving a source follower structure, for receiving the current signal toreduce an impedance of the signal source; a common source stage,following the source follower stage, driven by the reduced signal sourceimpedance, for amplifying the current signal to extend a frequencybandwidth of the current signal and buffering the amplified signal withthe extended frequency bandwidth thereof maintained, wherein the reducedsignal source impedance serves to extend a frequency bandwidth of thecommon source stage; and a shunt feedback resistor, which is installedbetween the source follower stage and the common source stage, foradjusting an input DC bias of the source follower stage and increasing atransimpedance gain of the common source stage.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The above and other objects and features of the present inventionwill become apparent from the following description of preferredembodiments given in conjunction with the accompanying drawings, inwhich:

[0013]FIG. 1 shows a circuit diagram of a conventional common sourcetransimpedance amplifier (TIA);

[0014]FIG. 2 depicts a circuit diagram of a source follower TIA inaccordance with a preferred embodiment of the present invention;

[0015]FIG. 3 is a high frequency model of a first amplifying part shownin FIG. 1;

[0016]FIGS. 4A and 4B offer a high frequency model of a source followerstage shown in FIG. 2 and an equivalent circuit diagram for describingoutput impedance thereof, respectively;

[0017]FIG. 5 provides graphs for illustrating a frequency bandwidth ofthe conventional common source TIA and the source follower TIA of thepresent invention, respectively; and

[0018]FIG. 6 presents graphs for illustrating a frequency bandwidth of aprior art shunt inductive-peaking TIA, a prior art capacitive-peakingTIA and the source follower TIA of the present invention, respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019]FIG. 2 describes a circuit diagram of a source followertransimpedance amplifier (TIA) 400 in accordance with a preferredembodiment of the present invention, which includes a plurality oftransistors 40, 110, 116, 120 and 126, a multiplicity of resistors 102,106, 108, 112, 114, 118, 122, 124, and 128 and a power supply 104.

[0020] The transistor 40, an input transistor which has a sourcefollower structure, in an input node of the source follower TIA 400forms a source follower stage of the source follower TIA 400 asrepresented by a dotted line and the transistors 110, 116, 120 and 126following the source follower stage form a common source stage of thesource follower TIA 400.

[0021] The input transistor 40 receives a current signal from a signalsource (not shown) and forwards the current signal to the common sourcestage. The input transistor 40 serves to drop a signal source impedanceseen across the common source stage. The signal source impedance seenacross the common source stage is smaller than that seen across theinput transistor 40.

[0022] The small signal source impedance serves to extend a frequencybandwidth. The effect of the signal source impedance will be describedlater.

[0023] A gate of the input transistor 40 is connected to a drain of thetransistor 110 via a shunt feedback resistor 108, while a drain of theinput transistor 40 is coupled to the power supply 104 via a resistor102 and a source of the input resistor 40 is grounded via a resistor106.

[0024] The shunt feedback resistor 108, which is installed between thegate of the input transistor 40 and a drain of the transistor 110,serves to adjust an input DC bias for a gate voltage of the inputtransistor 40.

[0025] The transistor 110 as a first amplifying stage basicallyamplifies the current signal forwarded from the source follower stage toextend a frequency bandwidth of the current signal. A gate of the firstamplifying transistor 110 is connected to the source of the inputtransistor 40, while a drain of the first amplifying transistor 110 iscoupled to the power supply 104 via a resistor 112 and a source of thefirst amplifying transistor 110 is grounded via the resistor 114.

[0026] Generally, because a single amplifying stage usually cannotprovide sufficient gain, other amplifying stage, e.g., a secondamplifying stage, may be required. In this case, the transistor 116 maybe prepared between the first amplifying stage and the second amplifyingstage, to function like as a first buffer stage for buffering thebasically amplified signal with the frequency bandwidth thereofmaintained. A gate of the first buffer transistor 116 is connected tothe drain of the first amplifying transistor 110, while a drain of thefirst buffer transistor 116 is coupled to the power supply 104 and asource of the first buffer transistor 116 is grounded via the resistor118.

[0027] The transistor 120 is used as the second amplifying transistorfor secondarily amplifying the buffered signal from the first buffertransistor 116, thereby extending a frequency bandwidth of the bufferedsignal. A gate of the second amplifying transistor 120 is connected tothe source of the first buffer transistor 116, while a drain of thesecond amplifying transistor 120 is coupled to the power supply 104 viathe resistor 122 and a source of the second amplifying transistor 120 isgrounded via the resistor 124.

[0028] The transistor 126 in an output node of the source follower TIA400 is used as a second buffer stage for delivering the secondarilyamplified signal with the frequency bandwidth thereof maintained andsimultaneously adjusting the output impedance of the source follower TIA400 to be a predetermined value, e.g., 50 Ω. A gate of the second buffertransistor 126 is connected to the drain of the second amplifyingtransistor 120, while a drain of the second buffer transistor 126 iscoupled to the power supply 104 and a source of the second buffertransistor 126 is grounded via the resistor 128.

[0029] In accordance with the present invention, the source followerstage is installed in front of the common source stage, thereby reducingthe source impedance seen across the common source stage. The reducedsource impedance affects the common source stage to output a signal withan extended frequency bandwidth.

[0030] Such characteristics will be described hereinafter with referenceto FIGS. 3 to 4B.

[0031]FIG. 3 is a high frequency model of a first amplifying part,driven by R_(S), represented by a dotted line in a conventional commonsource TIA 100 shown in FIG. 1, wherein R_(S) is a signal sourceimpedance seen across a gate of the transistor 10.

[0032] The first amplifying part in the conventional common source TIA100 has an input frequency magnitude ω_(in) given as follows:$\begin{matrix}{{\omega_{in} = \frac{1}{R_{S}\left\lbrack {C_{GS} + {\left( {1 + {g_{m}R_{D}}} \right)D_{GD}}} \right\rbrack}},} & {{Eq}.\quad 1}\end{matrix}$

[0033] wherein C_(GS) is a capacitance between a gate and a source ofthe transistor 10; C_(GD) is a capacitance between the gate and a drainof the transistor 10; g_(m) is a transconductance of the transistor 10;and R_(D) is a drain impedance of the transistor 10.

[0034] As seen from Eq. 1, the input frequency magnitude ω_(in) is ininverse proportion to the signal source impedance Rs. That is, the lessthe signal source impedance R_(S) is, the better the frequency bandwidthcharacteristics at the high-frequency range is.

[0035]FIG. 4A shows a high frequency model of a source follower stage,represented by a dotted line in FIG. 2, while FIG. 4B offers anequivalent circuit diagram of the high frequency model shown in FIG. 4Afor finding an output impedance of the source follower stage, wherein CLrepresents a total capacitance of the source follower stage seen at theoutput node of the high frequency model.

[0036] The source follower stage has the output impedance, which isconsidered at an output end of the high frequency model, as follows:$\begin{matrix}{{Z_{out} = \frac{{{sR}_{S}^{\prime}C_{GS}^{\prime}} + 1}{g_{m}^{\prime} + {sC}_{GS}^{\prime}}},} & {{Eq}.\quad 2}\end{matrix}$

[0037] wherein R_(S) is a signal source impedance seen across a gate ofthe transistor 40; C_(GS)′ is a capacitance between a gate and a sourceof the transistor 40; g_(m)′ is a transconductance of the transistor 40;and “s” is a complex frequency parameter.

[0038] The output impedance Z_(out) of the source follower stage isapproximately Z_(out)≈1/g_(m)′ in a low frequency range andZ_(out)≈R_(S) in an infinite frequency. And, 1/g_(m)′ is generally lessthan R_(S). When frequency is infinite in an ideal case, the outputimpedance Z_(out) of the source follower stage equals the signal sourceimpedance R_(S).

[0039] Because the source follower stage is installed in front of thecommon source stage including the first amplifying stage similar to afirst amplifying part shown FIGS. 1 and 3, the signal source impedanceseen across a gate of the first amplifying transistor 110 is to beZ_(out).

[0040] As seen from Eq. 2, the output impedance Z_(out) is alwayssmaller than R_(S) unless a frequency is infinite. Therefore, the sourcefollower stage renders that the first amplifying stage has a signalsource impedance smaller than that of the first amplifying part, therebyextending the frequency bandwidth of the common source stage.

[0041] When a transimpedance gain of the source follower TIA 400 isequal to that of the conventional common source TIA 100, the frequencybandwidth of the source follower TIA 400 is greater than that of theconventional common source TIA 100. The frequency bandwidth of thesource follower TIA 400 is also greater than those of a prior art shuntinductive-peaking TIA and a prior art capacitive-peaking TIA if theprior art shunt inductive-peaking, the prior art capacitive-peaking TIAand the source follower TIA 400 have same transimpedance gain. Suchfrequency bandwidth characteristics will be described hereinafter withreference to FIGS. 5 and 6.

[0042]FIG. 5 provides simulation results for illustrating frequencybandwidths of the conventional common source TIA 100 and the sourcefollower TIA 400 of the present invention, wherein the longitudinal axisrepresents a transimpedance gain [dBΩ] and the horizontal axisrepresents a frequency bandwidth [GHz].

[0043] For a transimpedance gain of 57 dBΩ, an available frequencybandwidth of the conventional common source TIA 100 is about 2.45 GHz asshown in a graph G₁₀₀, while an available frequency bandwidth of thesource follower TIA 400 is about 4.5 GHz as shown in a graph G₄₀₀. Thatis, the frequency bandwidth of the source follower TIA 400 of thepresent invention is larger than that of the conventional common sourceTIA 100 by about 2.05 GHz without any transimpedance gain loss.

[0044] In accordance with the present invention, the transimpedance gainof the source follower TIA 400 can be increased at the same frequencybandwidth by raising the resistance of the shunt feedback resistor 108of the source follower TIA 400 shown in FIG. 2. That is, a greatergain-bandwidth multiplication of the source follower TIA 400 of thepresent invention is larger than that of the conventional common sourceTIA 100.

[0045]FIG. 6 presents simulation results for illustrating frequencybandwidths of the prior art shunt inductive-peaking TIA, the prior artcapacitive-peaking TIA and the source follower TIA 400 of the presentinvention, wherein the longitudinal axis represents a transimpedancegain [dBQ] and the horizontal axis represents a frequency bandwidth[GHz].

[0046] For a transimpedance gain of 57 dBΩ, an available frequencybandwidth of the prior art shunt inductive-peaking TIA is about 2.85 GHzas shown in a graph G₂₀₀, while an available frequency bandwidth of theprior art capacitive-peaking TIA is about 3.25 GHz as shown in a graphsG₃₀₀. On the other hand, the source follower TIA 400 has an availablefrequency bandwidth of about 4.5 GHz for the transimpedance of 57 dBΩ asshown in a graph G₄₀₀.

[0047] That is, the gain-bandwidth multiplication of the source followerTIA 400 of the present invention is larger than those of the prior artshunt inductive-peaking TIA and the prior art capacitive-peaking TIA,without any transimpedance gain loss.

[0048] Meanwhile, the source follower TIA 400 using a FET (Field EffectTransistor) device having gate/source/drain has been described as theTIA in accordance with the present invention, but an emitter followerusing a BJT (Bipolar Junction Transistor) having base/emitter/collectormay be used.

[0049] Because no additional inductor and a capacitor are used therein,the source follower TIA 400 of the present invention has a smaller sizethan those of the prior art inductive-peaking TIA and the prior artcapacitive-peaking TIA and is immune to a self-resonant frequency (SRF)and Q-value of on chip components unlike the prior art inductive-peakingTIA and the prior art capacitive-peaking TIA.

[0050] While the invention has been shown and described with respect tothe preferred embodiments, it will be understood by those skilled in theart that various changes and modifications may be made without departingfrom the spirit and scope of the invention as defined in the followingclaims.

What is claimed is:
 1. A transimpedance amplification apparatus with asignal source for generating a current signal, which comprises: a sourcefollower stage having a source follower structure, for receiving thecurrent signal to reduce an impedance of the signal source; a commonsource stage, following the source follower stage, driven by the reducedsignal source impedance, for amplifying the current signal to extend afrequency bandwidth of the current signal and buffering the amplifiedsignal with the extended frequency bandwidth thereof maintained, whereinthe reduced signal source impedance serves to extend a frequencybandwidth of the common source stage; and a shunt feedback resistor,which is installed between the source follower stage and the commonsource stage, for adjusting an input DC bias of the source followerstage and increasing a transimpedance gain of the common source stage.2. The apparatus of claim 1, wherein the common source stage includes: afirst amplifying transistor for basically amplifying the current signal,wherein a gate thereof is connected to the source follower stage, whilea drain thereof is coupled to a power supply and a source thereof isgrounded; a first buffering transistor for buffering the basicallyamplified signal with the bandwidth thereof maintained, wherein a gatethereof is connected to the drain of the first amplifying transistor,while a drain thereof is coupled to a power supply and a source isgrounded; a second amplifying transistor for secondarily amplifying thebuffered signal, wherein a gate thereof is connected to the source ofthe first buffering transistor, while a drain thereof is coupled to apower supply and a source thereof is grounded; and a second bufferingtransistor for adjusting output impedance of an output signal from thecommon source stage to be a predetermined value and buffering thesecondarily amplified signal while maintaining frequency bandwidththereof, wherein a gate thereof is connected to the drain of the secondamplifying transistor, while a drain thereof is coupled to a powersupply and a source thereof is grounded.
 3. The apparatus of claim 2,wherein the shunt feedback resistor is installed between a gate of thesource follower stage and the drain of the first amplifying transistor.4. The apparatus of claim 3, wherein the predetermined value of theoutput impedance is about 50 Ohm.
 5. The apparatus of claim 2, whereinthe first amplifying transistor has an input frequency magnitude ω_(in)as follows:${\omega_{in} = \frac{1}{R_{S}\left\lbrack {C_{GS} + {\left( {1 + {g_{m}R_{D}}} \right)D_{GD}}} \right\rbrack}},$

wherein R_(S) is the signal source impedance; C_(GS) is a capacitancebetween the gate and the source of the first amplifying transistor;C_(GD) is a capacitance between the gate and the drain of the firstamplifying transistor; g_(m) is a transconductance of the firstamplifying transistor; and R_(D) is a drain impedance of the firstamplifying transistor.
 6. The apparatus of claim 5, wherein the sourcefollower stage has an output impedance as follows:${Z_{out} = \frac{{{sR}_{S}^{\prime}C_{GS}^{\prime}} + 1}{g_{m}^{\prime} + {sC}_{GS}^{\prime}}},$

wherein C_(GS)′ is a capacitance between the gate and the source of thesource follower stage; g_(m)′ is a transconductance of the sourcefollower stage; and “s” is a complex frequency parameter.
 7. Theapparatus of claim 6, wherein each transistor is a FET (Field EffectTransistor) device.
 8. The apparatus of claim 6, wherein each transistoris a BJT (Bipolar Junction Transistor) device.